Switching power supply devices are used as alternating current/direct current (AC/DC) converters or DC/DC converters. There have been switching power supply devices in which a primary-side circuit unit (in the case of an AC/DC converter, a circuit unit which receives power from an AC power source) and a secondary-side circuit unit (in the case of a DC/DC converter, a circuit unit which outputs a DC voltage) are electrically isolated from each other and magnetically coupled to each other via a transformer. Further, as a circuit that rectifies a voltage generated on a secondary winding of the transformer, a synchronous rectifier circuit in which a transistor connected to the secondary winding (hereinafter, also referred to as a secondary-side switch) is used to switch on and off the secondary-side switch at timings in accordance with the voltage waveform of the secondary winding is employed for rectification. Note that, in recent years, in order to accurately control secondary-side switches and further improve the conversion efficiency, control circuits such as a dedicated control integrated circuit (IC) are often used.
Operation modes in such a switching power supply device are categorized into a discontinuous current mode, a critical current mode, and a continuous current mode. The discontinuous current mode is an operation mode in which the primary-side and secondary-side switches are controlled such that the current flowing in the switching transistor included in the primary-side circuit unit (hereinafter, also referred to as a primary-side switch) and the current flowing in the secondary-side switch are both 0 A for a certain period in each cycle of the current waveform. The critical current mode is an operation mode in which the primary-side and secondary-side switches are controlled such that the currents of both the switches described above are 0 A at the same point of time. The continuous current mode is an operation mode in which the primary-side and secondary-side switches are controlled such that there is no period or point of time in which the currents of both the switches described above are 0 A. In the continuous current mode, a larger output current may be obtained than in other modes.
In the continuous current mode, however, when the primary-side switch is switched on while a current flows in the load direction through the secondary-side switch, a voltage with a large positive value may be applied to the terminal of the secondary-side switch connected to the secondary winding. At this time, when the secondary-side switch remains switched on due to operation delay thereof, a large current flows to the reference potential side through the secondary-side switch, which results in a large power loss.
In the related art, intentionally delayed operation of the primary-side switch is used to avoid simultaneous switching on of the primary-side switch and the secondary-side switch.
In such an art using intentionally delayed operation of the primary-side switch, a dead time during which both the primary-side switch and the secondary-side switch are switched off occurs. To realize the continuous current mode, a diode may be connected parallel to the secondary-side switch so that a current flows to the load side in the dead time. When a silicon (Si)-metal oxide semiconductor field effect transistor (MOSFET) is used as the secondary-side switch, a diode structurally included inside the Si-MOSFET (body diode) serves the function described above.
However, since the forward voltage of such a diode is larger than a voltage between both the terminals of the secondary-side switch when switched on, an increased dead time and thus a longer period during which a current flows in the diode cause a problem of a larger power loss.
The followings are reference documents.    [Document 1] Japanese Laid-open Patent Publication No. 09-285116,    [Document 2] Japanese Laid-open Patent Publication No. 11-235029, and    [Document 3] Japanese Laid-open Patent Publication No. 2003-333846.